1. Field of the Invention
The present invention relates generally to an organic electroluminescent device having at least a luminescent layer using an organic material provided between a hole injection electrode and an electron injection electrode, and more particularly, to an organic electroluminescent device capable of emitting light efficiently at a low voltage and stably over a long time period.
2. Description of the Prior Art
In recent years, the needs of plane display devices the consumed power and the spatial occupied area of which are smaller than those of a CRT which has been conventionally generally employed have been increased as information equipments are diversified, for example, whereby an electroluminescent device (hereinafter referred to as an EL device) has been paid attention to as one of the plane display devices.
The EL device is roughly divided into an inorganic EL device using an inorganic material and an organic EL device using an organic material depending on the used material.
The inorganic EL device is so adapted that a high electric field is generally exerted on a luminescent portion, and electrons are accelerated within the high electric field to collide with a luminescence center, whereby the luminescence center is excited to emit light.
On the other hand, the organic EL device is so adapted that electrons and holes are respectively injected into a luminescent portion from an electron injection electrode and a hole injection electrode, the electrons and the holes thus injected are recombined with each other in a luminescence center to excite an organic material, and the organic material emits fluorescence when it is returned from its excited state to its ground state.
In the inorganic EL device, a high voltage of 100 to 200 V is required as its driving voltage because a high electric field is exerted as described above. On the other hand, the organic EL device can be driven at a low voltage of approximately 5 to 20 V. Further, in the organic EL device, a device emitting light in a suitable color can be simply obtained by selecting a fluorescent material which is a luminescent material, and it is expected that the device can be also utilized as a full-color display device or the like. In recent years, various studies have been undertaken on the organic EL device.
Known examples of the organic EL device generally include ones having a three-layer structure which is referred to as a DH structure in which a hole transporting layer, a luminescent layer and an electron transporting layer are laminated between a hole injection electrode and an electron injection electrode, a two-layer structure which is referred to as an SH-A structure in which a hole transporting layer and a luminescent layer abundant in electron transporting characteristics are laminated between a hole injection electrode and an electron injection electrode, and a two-layer structure which is referred to as an SH-B structure in which a luminescent layer abundant in hole transporting characteristics and an electron transporting layer are laminated between a hole injection electrode and an electron injection electrode.
The organic EL device has the advantage that it can be driven at a lower voltage than the inorganic EL device as described above, so that it can be easily multi-colored, while having some disadvantages. For example, a voltage of approximately 5 to 20 V is still required to drive the organic EL device, and the organic EL device cannot be driven simply by a dry cell. Further, much of power which did not contribute to light emission is changed into heat. The device is degraded by the heat, whereby light is not emitted stably over a long time period.